Camera tube

ABSTRACT

An intensified television camera tube including a channel multiplier array incorporated adjacent the target electrode to multiply the electrons being emitted from the source, the channel multiplier array being utilized as a mechanical support for the target electrode and as the output screen or readout mesh. Also, a system for eliminating spurious signals from the output video signal which includes a capacitance network connected in circuit with the input and output sections of the multiplier array.

United States Patent [1 1 Catchpole Oct. 30, 1973 CAMERA TUBE [75]Inventor: Clive E. Catehpole, Southfield,

Mich.

[731 Assignee: The Bendix Corporation, Southfield,

Mich.

[22] Filed: Apr. 1, 1971 [21] Appl. No.: 130,440

Related US. Application Data [62] Division of Ser. No. 801,627, Feb. 24,1969,

abandoned,

[52] US. Cl. 315/12, 313/68 R [51] Int. Cl. H0lj 29/41 [58] Field ofSearch 315/10, 11, 12; 313/68 R [56] References Cited UNITED STATESPATENTS 3,350,591 10/1967 Van Asselt 313/65 T 7/1969 Shoulders 10/1970Maeda 313/104 X Wm [ix/MIX 1 3,039,017 6/1962 Brown et a1. 313/68 X3,202,853 8/1965 Weimer 313/65 3,440,470 4/1969 Decker 313/103 3,497,7592/1970 Manley 313/105 X Primary Examiner-Leland A. Sebastian AssistantExaminerJ. M. Potenza AttorneyPlante, Hartz, Smith & Thompson andWilliam F. Thornton [57] ABSTRACT 19 Claims, 6 Drawing Figures fir A /4CAMERA TUBE CROSS REFERENCE TO RELATED APPLICATIONS This is a divisionalof application Ser. No. 801,627 filed Feb. 24, 1969, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates generallyto an improved image intensifier tube and associated output circuit aridmore particularly to an image tube incorporating an electron multiplierassembly for amplifying the number of electrons striking the targetelectrode and for being utilized as a readout mesh, and support memberand, further, an output circuit modification for substantially reducingspurious noise signals from the readout video circuit.

The features of the present invention will be described as associatedwith an image orthicon type television camera tube, however, it is to beunderstood that these features may be utilized in other types ofsystems. In an image orthicon tube, an optical image is focused on aphotocathode, the photocathode being positioned immediately inside theglass envelope of the tube. The image focused on the photocathode causeselectrons to be emitted therefrom, the electrons traveling from thephotocathode to a target electrode located parallel to the photocathodeand spaced therefrom approximately 1 /2 inches. With the proper biasvoltages on certain elements, the electron image formed on thephotocathode is drawn to the target. The impinging electrons on thetarget electrode give rise to secondary emission of electrons, thesecondary electrons being collected in the prior art tubes, by a finemesh screen located adjacent the surface of the target and between thephotocathode and target electrodes. The target thereby becomes chargedwith a distribution proportional to the brightness of the optical image,and the value of this charge is several times greater than the impingingelectron image, by virtue of the electron multiplication which occurs inthe act of secondary emission of electrons at the target surface.

The target is made of very thin glass or other suitable material havinghigh resistivity. The lateral resistance of the target electrode issufficient to preserve the charge configuration on the target for theduration of the frame interval. The image may be sensed on the tar getby two methods of image removal, one being referred to as the returnbeam type of readout and the other the direct beam readout. In thereturn beam readout mode of operation, a scanning beam generated by anelectron gun is directed against the opposite side of the target fromthe mesh screen and the photocathode. The scanning beam creates a returnbeam directed back toward the electron gun and the amount of readoutbeam returning to the electron gun is measured by means of an electronmultiplier assembly positioned adjacent the electron gun. The amount ofbeam necessary to reestablish the target potential is, of necessity,missing or absent from the beam reflected from the target and thisabsent portion provides the video output signal. This return beamoperation is illustrated in the drawings.

The drawings further illustrated the direct beam readout operationwherein the target is scanned by an electron beam. During this scanning,the potential change of the target as it is returned to gun cathodepotential by the readout beam is sensed. This sensing is done, in theprior art by utilizing the readout mesh positioned between thephotocathode and the target electrodes as a signal plate. The potentialchanges on the target are capacitively coupled to the signal plate fordetection by a suitable amplifier in the video output circuit.

In accordance with the features of the present invention, an electronmultiplier device in the form of a channel multiplier has been providedin lieu of the fine mesh screen, the channel multiplier array beingutilized as the signal plate in this latter mode of readout operation,and as an electron multiplying system. Thus, the electron image isgreatly amplified over that of prior art tubes.

Further, in accordance with certain other features of the presentinventions, it is contemplated that the channel multiplier arrayadditionally be utilized as a mechanical support for the thin targetfilm, thus enabling the tube to withstand greater mechanical andelectrical shock. In using this feature, the target is placed directlyon the output portion of the multiplier array, thus being supportedthereby. Further, with the use of this feature, the effectivecapacitance of the storage target is increased, thereby enabling ahigher charge to be accumulated on the target electrode to give a higheroutput signal-to-noise ratio capability. Also, secondary electronsproduced at the target are physically constrained from spreading tocause redistribution effects known as black halo.

In accordance with certain other features of the present invention, thechannel multiplier array is provided with a film on the output sectionof the array to provide an insulating layer between the multiplier arrayand the target film, it being contemplated that this layer would be madeup of magnesium fluoride or other like materials. This layer has beenfound to be helpful in reducing possible spurious signals in utilizingthe channel multiplier array as a support structure. These signalsexhibit themselves because the positive voltage of the channelmultiplier array relative to the electron gun cathode causes a leakagethrough the storage film to appear as a signal on the image readout. Ifthe target film is perfectly uniform, the signal is uniform and may besubtracted from the picture information electronically. However, inpractical applications, it is not to be expected that the target filmwill be perfectly uniform, thus producing nonuniformities in the form ofpicture blemishes and shading.

Further, in utilizing a channel multiplier array to intensify the imagebeing transmitted to the target electrode, it has been found thatadditional spurious signals are generated in the output circuit due tothe presence of the array when it is operating with high electron gain.Accordingly, a spurious signal elimination circuit has been devised forsubstantially reducing the noise signals generated by the channelmultiplier array. The signal elimination circuit takes the form, in thepreferred embodiment, of a capacitor network interconnected between thetwo faces of the channel multiplier array to provide a shunt path forthe spurious signals, thus eliminating these signals from the outputcircuit.

Accordingly, it is one object of the present invention to provide animproved image intensified camera tube.

It is another object of the present invention to provide an improvedcamera tube having capability of am- 3 plifying the image whileeliminating the necessity for a readout screen within the camera.

It is a further object of the present invention to provide an improvedcamera tube incorporating an image intensifying system which may also beutilized as a readout mesh.

It is a further object of the present invention to provide an improvedcamera tube having improved mechanical and electrical shockcharacteristics.

A further object of the present invention is to provide an improvedcamera tube having improved charge distribution characteristics on thetarget electrode.

It is still a further object of the present invention to provide animproved camera tube wherein the thin target film is mechanicallysupported on a heretofore unused support structure.

It is a further object of the present invention to increase theeffective capacitance of the storage target electrode, thus enabling ahigher charge to be accumulated on the target to give a higher outputsignal-tonoise ratio capabiity.

It is still a further object of the present invention to provide acamera tube having means for physically constraining the secondaryelectrons produced at the target electrode.

It is a further object of the present invention to provide an improvedcamera tube wherein picture blemishes and shading are eliminated in thecase of a nonuniform target electrode.

It is still another object of the present invention to eliminatespurious signals from the output circuit of image camera tube.

And, it is a further object of the present invention to provide a cameratube having a channel multiplier array positioned adjacent the targetelectrode, the channel multiplier array being utilized as a mechanicalsupport for the target film, an electron multiplying device and areadout screen mesh.

Further objects, features and advantages of this invention will becomeapparent from a consideration of the following description, the appendedclaims and the accompanying drawing in which:

FIG. 1 is a schematic diagram illustrating portions of a camera tubeincorporating certain features of the present invention, the readoutbeing in the return beam mode of operation;

FIG. 2 is a schematic diagram similar to FIG. 1, but illustrating thedirect beam readout mode of operation;

FIG. 3 is a detail diagram illustrating the portion in circle A of FIG.1;

FIG. 4 is a modification of the details of FIG. 3;

FIG. 5 is a further modification of the details of FIG. 3; and

FIG. 6 is a schematic diagram illustrating a preferred form of spurioussignal eliminator circuitry incorporating certain features of thepresent invention.

Referring now to the drawing, in particular FIG. 1, there is illustrateda schematic diagram of certain portions of a camera tube 10 whichincludes a photocathode 12 on which an input optical image is focused,and a target electrode 14 on which the electron image is focused. Thetube 10 further includes an intervening electron multiplier array 16 forincreasing the number of electrons emitted from the photocathode priorto the impingement thereof on the target electrodes 14. The above notedelements of the camera tube are enclosed in an evacuated envelope, as iswell known in the art.

The photocathode is adapted to emit electrons in response to thefocusing of an image through the face of the camera tube and on thephotocathode surface. The electrons emitted from the photocathode arefocused, either magnetically, electrostatically, or by proximity, on tothe microchannel plate multiplier. The multiplied electrons emitted fromthe microchannel plate are incident on the charge storage target. Thetarget can be fabricated from a thin glass disc, typically 0.0001 inchthick. Other materials can also be used for a charge storage target. Theprinciples of operation of the tube as described here with reference toa charge storage target made from a thin glass disc have been shown tohold for charge storage targets made from thin films of Aluminum Oxideand Magnesium Oxide, typically 500A thick. The principle has also beenshown to be valid for charge storage layers made from insulatingmaterials, such as Potassium Chloride or Magnesium Fluoride, evaporatedin a gaseous atmosphere to form a porous spongy layer about 10 micronsthick. These, and other, methods of charge storage target preparationare well known in the art. The surface of the target 14 is, in certainembodiments of the invention, supported with its surface approximately0.002 inches away from the output of the channel multiplier array 16.Thus, the electrons being emitted from the photocathode pass into thechannel array 16, are amplified by the secondary emission within thechannel multiplier array, and the output electrons strike the targetelectrode 14.

The output electrode of the array is normally held at a positive biaspotential; the value of this potential depends upon the target materialused, but a potential of the order of 10 volts is usually satisfactory.The electrons emerging from the microchannel array have the propertythat they have sufficient energy to pass the potential barrier betweenthe array and the target and still have sufficient energy to liberate onthe average of more than one secondary electron from the target. Thesecondary electrons are collected by the multiplier array outputelectrode, to be discussed more in detail in conjunction with thedescriptions in FIGS. 3 to 5, thus leaving the target with a positivelycharged image. This positive charge is capacitively coupled to theopposite side of the target and hence varies the electron scanning beamconventionally utilized to scan the target 14.

The scanning beam is generated from an electron emitting gun 20 whichsupplies electrons towards the target 14 in a direct beam. For purposesof discussion, the return beam operation mode of FIG. 1 will bedescribed first and the direct beam mode of operation will be describedin conjunction with the description of FIG. 2. The electron scanningstream is directed through a multiplier array 22, in the form of adynode structure, and thence impinges on the target electrode 14. If thetarget electrode requires electrodes to return it to its normalpotential, due to the charge capacitance coupled from the chargedsurface on the opposite side of the target electrode, a certain portionof the direct beam will be utilized in furnishing these electrons. Theremaining portion will return to the electron gun and will beintercepted by the dynode structure 22. The dynode 22 includes aplurality of plates 24 which are utilized to multiply the electronsbeing fed therethrough, and the output from the dynode structure isderived from a video output conductor 26.

A similar operation exists in the system of FIG. 2, which also includesa photocathode 12, a channel multiplier array 16, and a target electrode14. Further, a stream of scanning electrons are supplied by an electrongun 20. However, the charge on target electrode 14 is sensed bycapacitive coupling between the target electrode 14 and a conductiveoutput face of the channel multiplier array 16, as will be more fullyexplained in connection with the explanation of FIGS. 3 to 5. The outputfrom the conductive surface of the channel multiplier array 16 is fed toa video output circuit, including a coupling capacitor 28 and a resistor30, the latter of which is connected to a positive voltage biasingsource. This operation is conventionally known as the direct beamreadout operation.

In this direct beam readout mode of operation, the potential change ofthe target, as it is returned to gun cathode potential by the read-outscanning beam, is detected by a signal plate. In the case of the systemof the present invention, this signal plate takes the form of aconductive surface on the output side of the channel output multiplierarray 16. In the case of prior art tubes, the "signal plate takes theform of a readout mesh screen. The potential changes on the target arecapacitively coupled to the signal plate, the output electrode beingconnected with a suitable amplifier to amplify the video signal.

Referring now to FIGS. 3, 4, and 5, there are illustrated threevariations of the channel multiplier arrays which may be utilized inconjunction with the systems of FIGS. 1 and 2. Specifically, the targetelectrode 14 is mounted spaced, in the first embodiment, from thechannel multiplier array. The array takes the form of a body member 34fabricated of glass or other suitable material, the output end of whichis rendered conductive by means of a conductive coating 36 formedthereon. It is to be understood that the conductive coating 36 iscoextensive with the output end of the channel multiplier array 16. Inthe case of FIG. 3, the target electrode 14 must be suitablymechanically and electrically mounted on some structure to withstandshocks of a preselected degree.

Referring to FIG. 4, there is illustrated the channel multiplier array16 of FIG. 3 with the target electrode 14 supported directly on theconductive portions of the output end of the channel multiplier array16. In this way, the target electrode 14 is rigidly supported by thearray to permit it to withstand greater mechanical and electrical shockthan prior camera tubes were capable of withstanding. Further, theeffective capacitance of the storage target is greatly increased,thereby enabling a higher charge to be accumulated on the target, withthe result of an improved signal-to-noise ratio capability. Further,secondary electrons produced at the target are physically constrainedfrom spreading, thus reducing redistribution effects.

FIG. 5 illustrates a channel multiplier array 16 which incorporates thedesirable features of both the assemblies of FIGS. 3 and 4. In FIG. 5,the body member 34 is provided with a conductive coating 36 at theoutput end thereof, as was the case with FIGS. 3 and 4. However, thearray 16 is also provided with an insulating coating 40, the insulatingcoating 40 providing a spacing between the conductive coating 36 and thetarget electrode 14. With this arrangement, it has been found that thedesirable features of FIG. 3, also incorporate the desirable features ofFIG. 4 including confining the secondary electron emission, highercapacitance capabilities and greater mechanical and electrical shockwithstanding characteristics. Additionally, the arrangement shown inFIG. 5 is advantageous when a target is used which relies for continuousoperation on the conductivity between its faces, as is the case when atarget made from a thin glass film is used. The advantage is that thepositive potential necessary on the microchannel plate output electrode36 for proper collection of the secondary electrons from the target isnot able to leak through to the target thence to the scanned surface ofthe target, to produce a spurious positive potential.

In utilizing a channel multiplier array of the type described above, ithas been discovered that spurious signals are generated at the outputterminal due to the electron travel down the channel, which causesdisplacement current to be intercepted by a channel multiplier plateoutput electrode. This output signal appears as a negative polaritysignal. Thus appearing as random white spots on the television monitorscreen. The number of these spots is proportional to the input signallevel to the photocathode. The spurious signal producing these whitespots is only evident when the microchannel plate multiplier isoperating with high electron gain. The magnitude of the spurious signalis proportional to the electron gain of the microchannel plate. When themicrochannel plate is operating with low gain, the spurious signals aresufficiently small that they are masked by noise sources in the system.In previous systems, the equal and opposite positive signal generatedalong the channel walls was not coupled to the video output terminal dueto the distributed capacitance and resistance along the channel. Inorder to substantially eliminate these spurious signals, the circuit ofFIG. 6 was devised to preclude any changes in charge distribution alongthe channels from being coupled into the video output circuit.

In the improved video output circuit, the output terminal 48 is coupledto the array through a coupling capacitor 50 similar to that describedin conjunction with FIG. 2, the other terminal of the capacitor 50 beingconnected to the output side of the channel multiplier array. Biasvoltage for the system is provided through a first resistor 52 and asecond resistor 54 which are connected to a bias conductor 56, the biasconductor 56 being connected to a source of positive DC potential. Thedistributed capacitance and resistance generated signal is shunted bymeans of a capacitor 60 and resistor 62 combination which are added tothe prior circuit to couple the equal and opposite positive signalgenerated along the channel walls to the video output.

The channel multiplier plate electrodes are connected together in analternating current sense by the capacitor 60 to form in effect aFaraday cage, and the input electrode is decoupled by the resistor 62.In practice, the capacitor 60 may be the capacitor plates formed by thetwo electrodes of the channel multiplier array, i.e., the input andoutput electrodes. One combination of capacitor 60 and resistor 62 whichhave been found to provide satisfactory results are where the input andoutput electrodes of the channel multiplier array, i.e., the input andoutput electrodes provide a ten picofarad capacitance and a resistor wasprovided in a range of approximately 470 Kilohms. However, it is to beunderstood that both resistor 62 and the capacitor 60 may have a widerange of values as long as the frequency l/CR is much lower than thehigh frequency cutoff of the video amplifier circuit. Actual choice ofcircuit parameters will be determined by consideration of space,convenience and loading of other circuit parameters.

While it will be apparent that the embodiments of the invention hereindisclosed are well calculated to fulfill the objects of the invention,it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope or fair meaning of the subjoined claims.

I claim:

1. In a camera tube system for deriving an output signal in response toan electron image of the type having an electron emissive member and atarget member separated therefrom, and an electron gun for scanning saidtarget member, the improvement comprising means for amplifying theelectron emission and deriving an output signal in response to thescanning of said target comprising:

an electron multiplier disposed between said emissive member and saidtarget member, said electron multiplier having an input face and anoutput face, an output conductive surface formed on said output face andan input conductive surface on said input face,

said target member being a charge storage layer positioned in theproximity of said output conductive surface so that said target memberand said conductive surface are capacitively coupled;

output terminal means connected to said conductive surface for receivingvoltages created on said conductive surface by said capacitive coupling;

and detection circuit means responsive to said voltages and yieldingsaid output signal, said detection circuit means including meansproviding capacitive coupling between said input conductive surface andsaid output conductive surface.

2. Th improvement of claim 1 wherein said target member is spaced fromsaid conductive surface.

3. The improvement of claim 1 wherein said conductive surface physicallysupports said target member.

4. The improvement of claim 3 further including an insulating layerbetween said conductive surface and said target member, said insulatinglayer being contiguous with both said conductive surface and said targetmember.

5. The improvement of claim 1 wherein said coupling is an alternatingcurrent coupling.

6. The improvement of claim 1 wherein said electron multiplier is achannel multiplier array having a body member, a plurality of channelsformed in said body member and extending between said input surface andsaid output surface, and the interior surface of each of said channelsbeing covered with electron emissive material.

7. The improvement of claim 6 wherein said target member is spaced fromsaid conductive surface.

8. The improvement of claim 6 wherein said conductive surface physicallysupports said target member.

9. The improvement of claim 8 further including an insulating layerbetween said conductive surface and said target member, said insulatinglayer being contiguous with both said conductive surface and said targetmember.

10. The improvement of claim 1 wherein said detection circuit furtherincludes impedance means electrically coupled to said input and outputconductive surfaces for shunting spurious signals produced at saidoutput surface by a high gain operation of said electron multiplier.

11. The improvement of claim 10 wherein said impedance means isconnected to said input conductive surface for electrically isolatingsaid input surface from ground so that the potential of said inputsurface can vary in response to shunted spurious signals and therebyeliminate said spurious signals.

12. The improvement of claim 11 wherein said impedance provides saidcapacitive coupling between said input conductivesurface and said outputconductive surface, and said impedance means includes resistive meansconnected to said input conductive surface for isolating said spurioussignals from said detection circuit means.

13. The improvement of claim 12 wherein said capacitive coupling isprovided by the capacitance inherently formed by said input and outputconductive surfaces.

14. In a camera tube system for deriving an output signal in response toan electron image of the type having an electron emissive member atarget member separated therefrom, and an electron gun for scanning thetarget member, the improvement comprising means for amplifying theelectron emission and deriving an output signal in response to thescanning of the target comprising:

an electron multiplier array disposed between said emissive member andsaid target member, said electron multiplier having an input face and anoutput face, an output conductive surface formed on said output face;

output terminal means coupled to said conductive surface for receivingimage signals created on said conductive surface;

and output detection circuit means responsive to said image signals andyielding said output signal.

15. The camera tube of claim 14 wherein said elec tron multiplierfurther includes an input conductive surface on said input face, andsaid detection circuit means includes means providing capacitivecoupling between said input conductive surface and said outputconductive surface.

16. The camera tube of claim 15 wherein said detection circuit furtherincludes impedance means electrically coupled to said input and outputconductive surfaces for shunting spurious signals produced at saidoutput surface by a high gain operation of said electron multiplier.

17. The camera tube of claim 16 wherein said impedance means isconnected to said input conductive surface for electrically isolatingsaid input surface from ground so that the potential of said inputsurface can vary in response to shunted spurious signals and therebyelimimate said spurious signals.

18. The camera tube of claim 17 wherein said impedance provides saidcapacitive coupling between said input conductive surface and saidoutput conductive surface, and said impedance means includes resistivemeans connected to said input conductive surface for isolating saidspurious signals from said detection circuit means.

19. The camera tube of claim 18 wherein said capacitive coupling isprovided by the capacitance inherently formed by said input and outputconductive surfaces. =l

1. In a camera tube system for deriving an output signal in response toan electron image of the type having an electron emissive member and atarget member separated therefrom, and an electron gun for scanning saidtarget member, the improvement comprising means for amplifying theelectron emission and deriving an output signal in response to thescanning of said target comprising: an electron multiplier disposedbetween said emissive member and said target member, said electronmultiplier having an input face and an output face, an output conductivesurface formed on said output face and an input conductive surface onsaid input face, said target member being a charge storage layerpositioned in the proximity of said output conductive surface so thatsaid target member and said conductive surface are capacitively coupled;output terminal means connected to said conductive surface for receivingvoltages created on said conductive surface by said capacitive coupling;and detection circuit means responsive to said voltages and yieldingsaid output signal, said detection circuit means including meansproviding capacitive coupling between said input conductive surface andsaid output conductive surface.
 2. Th improvement of claim 1 whereinsaid target member is spaced from said conductive surface.
 3. Theimprovement of claim 1 wherein said conductive surface physicallysupports said target member.
 4. The improvement of claIm 3 furtherincluding an insulating layer between said conductive surface and saidtarget member, said insulating layer being contiguous with both saidconductive surface and said target member.
 5. The improvement of claim 1wherein said coupling is an alternating current coupling.
 6. Theimprovement of claim 1 wherein said electron multiplier is a channelmultiplier array having a body member, a plurality of channels formed insaid body member and extending between said input surface and saidoutput surface, and the interior surface of each of said channels beingcovered with electron emissive material.
 7. The improvement of claim 6wherein said target member is spaced from said conductive surface. 8.The improvement of claim 6 wherein said conductive surface physicallysupports said target member.
 9. The improvement of claim 8 furtherincluding an insulating layer between said conductive surface and saidtarget member, said insulating layer being contiguous with both saidconductive surface and said target member.
 10. The improvement of claim1 wherein said detection circuit further includes impedance meanselectrically coupled to said input and output conductive surfaces forshunting spurious signals produced at said output surface by a high gainoperation of said electron multiplier.
 11. The improvement of claim 10wherein said impedance means is connected to said input conductivesurface for electrically isolating said input surface from ground sothat the potential of said input surface can vary in response to shuntedspurious signals and thereby eliminate said spurious signals.
 12. Theimprovement of claim 11 wherein said impedance provides said capacitivecoupling between said input conductive surface and said outputconductive surface, and said impedance means includes resistive meansconnected to said input conductive surface for isolating said spurioussignals from said detection circuit means.
 13. The improvement of claim12 wherein said capacitive coupling is provided by the capacitanceinherently formed by said input and output conductive surfaces.
 14. In acamera tube system for deriving an output signal in response to anelectron image of the type having an electron emissive member a targetmember separated therefrom, and an electron gun for scanning the targetmember, the improvement comprising means for amplifying the electronemission and deriving an output signal in response to the scanning ofthe target comprising: an electron multiplier array disposed betweensaid emissive member and said target member, said electron multiplierhaving an input face and an output face, an output conductive surfaceformed on said output face; output terminal means coupled to saidconductive surface for receiving image signals created on saidconductive surface; and output detection circuit means responsive tosaid image signals and yielding said output signal.
 15. The camera tubeof claim 14 wherein said electron multiplier further includes an inputconductive surface on said input face, and said detection circuit meansincludes means providing capacitive coupling between said inputconductive surface and said output conductive surface.
 16. The cameratube of claim 15 wherein said detection circuit further includesimpedance means electrically coupled to said input and output conductivesurfaces for shunting spurious signals produced at said output surfaceby a high gain operation of said electron multiplier.
 17. The cameratube of claim 16 wherein said impedance means is connected to said inputconductive surface for electrically isolating said input surface fromground so that the potential of said input surface can vary in responseto shunted spurious signals and thereby elimimate said spurious signals.18. The camera tube of claim 17 wherein said impedance provides saidcapacitive coupling between said input conductive surface and saidoutput conductive surface, and said impedance means includes resistivEmeans connected to said input conductive surface for isolating saidspurious signals from said detection circuit means.
 19. The camera tubeof claim 18 wherein said capacitive coupling is provided by thecapacitance inherently formed by said input and output conductivesurfaces.